There are 2 major requirement for a gasoline engine to fire, fuel and ignition. Since you have spark, then ignition is the least likely problem. No fuel could be verified by removing the air cleaner and pouring a small amount of fuel down the throat, start the bike, if it start and runs (even for just a moment), you have verified the fuel problem.
Problems then could be traced along the fuel flow from the ank all the way to the carburator. In some instances of run out of fuel, the bike will have a hard time starting due to:
1. air bubbles in the fuel lines;
2. valve in carb stuck in close position;
3. flooding of both carb and intakes;
4. (in some cases) vacuum insifficient to open petcock or fuel valve/switch.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.
Good luck and kind regards.
Thank you for using FixYa.
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